Polyesters including polyethylene terephthalate are excellent in mechanical strength, chemical stability, gas barrier properties and hygienic properties, and relatively inexpensive and lightweight. They have been therefore widely used as packaging containers for various foods and drinks.
When these packaging containers are produced, methods are used in which preformed articles are first formed and then heated again to soften them, followed by reforming to desired shapes. For example, when stretch blow molded products are produced, closed-end tubular preforms are first produced by injection molding, and these preforms are usually heated again with near infrared heaters to soften them. Thereafter, the softened preforms are allowed to come into contact with metal molds having specified shapes, thereby producing the blow molded products. However, these methods have the problem that productivity is poor because it takes time to heat the preforms again.
For solving such a problem, U.S. Pat. No. 5,419,936 proposes a packaging material comprising a thermoplastic polymer composition comprising a polymer which contains metal particles intrinsically absorbing radiation in the wavelength region 500 nm to 2000 nm in such an amount that the reheat time of the polymer is less than the reheat time of the polymer in the absence of the particles. Here, the radiation in the wavelength region 500 nm to 2000 nm is a radiation generated by a general near infrared heater. The above U.S. patent further discloses in the working examples a method of adding a trivalent phosphorus compound to a polyethylene terephthalate prepolymer as a reducing agent, 10 minutes after that, adding antimony trioxide as a reducible metal compound, followed by polymerization, and reacting the metal compound with the reducing agent to generate fine metal particles. However, a result of detailed studies by the present inventors has revealed that this method shows a large variation in size of particles generated and gives a considerably large number of coarse particles. The coarse particles are smaller in surface area than fine particles having the same volume, so that the radiation absorption efficiency is insufficient. Accordingly, when this composition is formed into a packaging material, the transparency thereof is poor. Further, when this composition is formed into a packaging material, the coarse particles raise the color tone and external problems that the packaging material strongly inclines to black, and that the coarse particles are observed as black foreign matter, because the coarse particles are metal particles, which are tinged with black. Furthermore, it is necessary to add a large amount of a relatively expensive trivalent phosphorus compound, because of the insufficient radiation absorption efficiency.
On the other hand, JP-B-49-20638 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a method for producing a polyester which comprises using a reaction solution in which metallic antimony is precipitated by allowing phosphorous acid, hypophosphorous acid, or an ester or salt thereof to act on a solution or suspension of an antimony compound as a polycondensation catalyst. However, as a result of detailed studies in this method by the present inventors, the ratio of coarse particles to precipitated particles contained in a polyester obtained by this method is high, and the radiation absorption efficiency is insufficient. Accordingly, when this polyester is formed into a packaging material, problems are encountered with regard to transparency and color tone.